An avalanche struck a rally of up to 200 snowmobilers in Canada's Rocky Mountains, killing at least three people and leaving an unknown number missing, police said.

Rescuers were still scouring remote Boulder Mountain, near Revelstoke, British Columbia, early today, the Royal Canadian Mounted Police said.

The RCMP said a dozen people were injured in the slide. Up to 200 people were on the mountain at the time for the annual Big Iron Shoot Out rally.

The RCMP said they did not yet have details about the three dead or the extent of the injuries. Nor did they know how many might be missing following the slide near Revelstoke, about 300km (185 miles) west of Calgary and about 400km (250 miles) north-east of Vancouver, late yesterday.

"The area has been shut down to keep people out," RCMP Corporal Dan Moskaluk said. "They're going to continue on here with trying to remove people off the mountain in the area and reassess where we're at.

"There's air assets in the form of helicopters ... RCMP police dogs, services dogs, avalanche trained dogs, a variety of machines and people, essentially it's a large scale rescue that's being coordinated," Cpl Moskaluk said.

Kathy Berlingette, owner of Smokey Bear Campground Resort in the area, said the event was in a remote place and everyone involved had to drive their snowmobiles out to get there.

The Canadian Avalanche Centre issued an avalanche warning for the region, which includes Revelstoke, for Saturday and today, after a powerful storm blanketed the region with snow.

Greg Johnson, from the avalanche centre, said at the time that there was danger that the snow would overload weak layers in the upper snowpack.

Revelstoke Mayor David Raven said the rally was an unauthorised event.

"The Canadian Avalanche Centre based in Revelstoke has had a warning for the last three weeks expressing extreme caution in the backcountry, a fresh snowfall overnight exacerbated that warning, I know people have been cautioned again and again," Mr Raven said.